1. Field of the Invention
The present invention relates to an apparatus for bevelling the edge of a semiconductor wafer (hereinafter, referred to as a wafer) by polishing.
2. Description of the Related Art
FIG. 17 is a plan view of a main structure of a prior-art wafer edge bevelling machine. The main structure of the prior-art wafer edge bevelling machine comprises an arm 101 an intermediate of which is mounted on a pivot 102, a cylindrical buff 103 rotatably mounted to an end of the arm 101, an air cylinder assembly 109 having a piston rod in contact with the side of the other end of the arm 101 for pushing the buff 103 against the edge of a wafer W, and a suction turntable 110 positioned near the buff 103 and sucking the wafer W in place.
In operation, a driver (not shown) rotates the buff 103 at a high speed counterclockwise in FIG. 17, the air cylinder assembly 109 concurrently pushes the other end of the arm 101 by a fixed force F in the direction of an arrow F, and the suction turntable 110 is rotated at a low speed counterclockwise in FIG. 17. Thus, the buff 103 is pressed on the edge of the wafer W by a contact pressure .sigma. and edge the edge of the wafer W is edged.
As shown in FIG. 5, the edge of the wafer W generally comprises an orientation flat W1, a circular edge W2, and round joints W3 between the orientation flat W1 and the circular edge W2. Since radii of curvature R1(=.infin.), R2 and R3 of the orientation flat W1, the circular edge W2 and the round joints W3 have a relational expression: R.sub.3 &lt;R.sub.2 &lt;R.sub.1, the areas C1, C2 and C3 of spots of contact between the buff 103 and each of the orientation flat W1, the circular edge W2 and the round joints W3 have a relational expression: C1&lt;C2&lt;C3.
Therefore, when the air cylinder assembly 109 pushes the side of the other end of the arm 101 by the fixed force F, the contact pressures .sigma..sub.1, .sigma..sub.2 and .sigma..sub.3 produced on the orientation flat W1, the circular edge W2 and the round joints W3 are reduced in response to the radii of curvature R1, R2 and R3 (i.e., .sigma..sub.1 &lt;.sigma..sub.2 &lt;.sigma..sub.3). In particular, the round joints W3 receive an excessive contact pressure so that the edges of the round joints W3 may have an abnormally strong grip on the buff 103 to abnormally polish or bevel the round joints W3.
Thus, the force F of the air cylinder assembly 109 must be so small that the round joints W3 will not have the abnormally strong grip on the buff 103. This reduces the productivity in the wafer edge bevelling.
Since the contact pressures .sigma..sub.1, .sigma..sub.2 and .sigma..sub.3 on the orientation flat W1, the circular edge W2 and the round joints W3 are different, the prior-art wafer edge bevelling machine cannot have an optimal uniform finish precision over the edge of the wafer W.
Since bevelling the edges of many wafers W wears a formed groove 103a in the buff 103 for bevelling the edge of wafer W to deteriorate or deform the section of the formed groove 103a, i.e., produce a permanent set in fatigue on the section of the formed groove 103a, a new formed groove 103a must be produced at a convenient time.
Conventionally, there are two methods of producing the formed groove in the buff. It is a first method that the buff 103 is removed from the wafer edge bevelling machine and then worked by a dedicated lathe to provide a new formed groove 103a. It is a second method that as shown in FIG. 18 a rotating shaft 110R of the wafer suction turntable 110 has a radially extending. Cutter 131 normally installed thereon and the cutter 131 is vertically moved and produces the formed groove 103a in the buff 103, if necessary.
In the first method, the attachment and the detachment of the buff 103 to and from the wafer edge bevelling machine are troublesome and the installation precision of the buff 103 to the wafer edge bevelling machine is problem. In the second method, the wafer edge bevelling machine produces the formed groove 103a while the rotating shaft 130 has the buff 103 normally installed thereon, so that the installation precision of the buff 103 on the rotating shaft 130 is not problem. However, the cutter 131 is normally exposed to a slurry 113 containing a polishing material during wafer edge bevelling because of the normal installation of the cutter 131 on the rotating shaft 130, so that the cutter 131 must be made of a special protective material, e.g., an expensive and difficultly available ceramic material. Therefore, the kinds of available cutters are restricted.